Electroplating method for producing ultralow-mass fissionable deposits

ABSTRACT

A method for producing ultralow-mass fissionable deposits for nuclear reactor dosimetry is described, including the steps of holding a radioactive parent until the radioactive parent reaches secular equilibrium with a daughter isotope, chemically separating the daughter from the parent, electroplating the daughter on a suitable substrate, and holding the electroplated daughter until the daughter decays to the fissionable deposit.

BACKGROUND OF THE INVENTION

The invention which is the subject of this application was created under a contract with the U.S. Department of Energy.

This invention relates to preparation of fissionable deposits and, more particularly, to an electroplating method for producing ultralow-mass fissionable deposits for nuclear reactor dosimetry.

Ultralow-mass fissionable deposits have proved useful as fissioning sources for solid state track recorder fission rate measurements in high intensity neutron fields. These fission rate measurements are used to derive information for neutron dosimetry purposes.

More particularly, a solid state track recorder placed adjacent a thin fissionable deposit records tracks from the recoiling fission fragments which result from the fissions in the deposit. The number of these tracks observed with an optical microscope after chemical etching of the solid state track recorder is proportional to the number of fissions that has occurred in the fissionable deposit. Thus, the number of fission fragment tracks per square centimeter, i.e., the track density, in the solid state track recorder can be used to calculate the fission rate per unit area in the fissionable deposit.

For typical high neutron fluence applications, such as reactor core dosimetry or reactor component dosimetry, it has been found that a limitation is placed on the use of solid state track recorders due to the maximum track density that can be used, usually about 10⁶ tracks/cm², without excessive track overlap. In order to avoid excessively high track densities, low-mass fissionable deposits have been used to reduce the number of fissions that will occur at a given neutron fluence. For example, in dosimetry applications for liqht water reactor pressure vessel surveillance, ²³⁹ Pu deposits with masses as low as 10⁻¹³ gram are required to produce a usable track density in the solid state track recorder. Similarly low masses of other isotopes such as ²³⁵ U, ²³⁸ U and ²³⁷ Np are required for dosimetry in light water reactor pressure vessel surveillance.

It has been found that the technical problems associated with the manufacture of such low-mass deposits can be overcome by using isotopic spiking/electroplating techniques to characterize the masses of these fissionable deposits. For example, low-mass deposits can be produced by an electroplating technique using ²³⁷ U (7 day half-life) as an isotopic spike for ²³⁵ U and ²³⁸ U, ²³⁹ Np (2.4 day half-life) as a spike for ²³⁷ Np, and ²³⁶ Pu (2.85y half-life) as a spike for ²³⁹ Pu. Typically, the shorter half-life isotopic spike is used as a chemical tracer to overcome the fact that the radioactivity of the respective fissionable deposit renders the principal isotope undetectable when present in such low masses as can.be employed according to the present invention.

However, it has also been found that the amount of the isotopic spike that can be added to a fissionable deposit is limited by the nuclear properties of the particular isotopic spike chosen. For example, ²³⁷ U decays to ²³⁷ Np and ²³⁹ Np decays to ²³⁹ Pu, each of which is also fissionable. In both cases, the amount of the isotope to which the spike eventually decays must be kept small enough (by limiting the amount of spike added) to keep the fission rate of the isotope to which the spike decays small relative to the decay rate of the isotope of interest in the deposit.

In particular regard to ²³⁹ Pu deposits, the ²³⁶ Pu isotopic spike itself is fissionable and must therefore be used in limited amounts. In addition, for ²³⁹ Pu deposits spiked with ²³⁶ Pu, several experimental problems arise. For example, in the case of a solid state track recorder using ²³⁹ Pu to measure the fission rate at the mid-plane location of the reactor cavity in the annular gap of an operating commercial power nuclear reactor during a typical operating cycle, a ²³⁹ Pu fissionable deposit with a mass of about 10⁻¹⁰ gram is required to produce an optimum number of fission tracks. Namely, due to the previously explained spiking limitations, the maximum allowable ²³⁶ Pu/²³⁹ Pu spike ratio for such a mass of the deposit results in a count rate that is only about 1 disintegration per minute (dpm).

In order to desirably characterize the decay rate of this deposit to better than 2% for mass calibration purposes, a relatively long counting time of about four days is required. Also, due to the low sample count rate, counters with very low background count rates (e.g., 0.1 dpm) must be used. However, the decay properties of the isotopic spike make maintenance of the low backgrounds difficult. For example, ²³⁶ Pu decays as follows: ##STR1## Thus, many radioactive decay products accumulate from the decay of ²³⁶ Pu, which must be periodically removed from the counters by cleaning to maintain low counter backgrounds.

On the other hand, when electroplating is employed to produce ²³⁷ Np deposits, it has been found that a ²³⁹ Np tracer can be obtained by milking a ²⁴³ Am source, and this tracer material must be chemically equilibrated with the ²³⁷ Np. The chemical equilibration, which requires a series of chemical oxidations and reductions using HBr and HNO₃, is necessary to ensure that the ²³⁷ Np and ²³⁹ Np are in the same equilibrium distribution of oxidation states and will therefore electroplate at the same rate, thusly quaranteeing that the ²³⁹ Np/²³⁷ Np ratio is a constant. After characterization of the ²³⁷ Np deposits via their ²³⁹ Np beta activity, a measurement of the ratio of beta activity to ²³⁷ Np alpha activity must be made to establish the mass scale for the deposits.

In light of the above, a simpler and more effective method is needed for producing ultralowmass fissionable deposits for nuclear reactor dosimetry.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide an electroplating method for producinq ultralow-mass fissionable deposits capable of easier deposit characterization.

It is another object of the present invention to provide a electroplating method for producing ultralow-mass fissionable deposits which are isotopically pure.

It is another object of the present invention to provide an electroplating method for producing ultralow-mass fissionable deposits wherein detector background is no longer a problem.

It is another object of the present invention to provide an electroplating method for producing ultralow-mass fissionable deposits wherein chemical equilibrium of tracers or spikes is unnecessary.

It is another object of the present invention to provide an electroplating method for producing ultralow-mass fissionable deposits capable of producing extremely low masses.

Finally, it is an object of the present invention to provide an electroplating method for producing ultralow-mass fissionable deposits capable of significant time savings in comparison with other methods.

To achieve the foregoing and other objects of the present invention, and in accordance with the purposes of the invention, there is provided a method for producing ultralow-mass fissionable deposits for reactor dosimetry, including the steps of holding a radioactive parent until the radioactive parent reaches secular equilibrium with a daughter formed thereby, chemically separating the daughter from the parent, electroplating the daughter on a suitable substrate, and holding the electroplated daughter until the daughter decays to the fissionable deposit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of the method of the present invention will now be described which is directed to producing a fissionable deposit of isotopically pure ²³⁹ Pu.

According to this embodiment, isotopically pure ²⁴³ Am is used as a radioactive parent source to prepare an isotopically pure ²³⁹ Np daughter. Preferably, the parent is a nuclide that decays by beta emission and has a half-life shorter than the daughter. An important consideration in determining the parent source is the ultimate daughter of which a low-mass fissionable deposit is sought.

The radioactive parent source is held until it reaches secular equilibrium with the daughter formed thereby. That is, ²⁴³ Am decays according to the following Equation (1): ##STR2## The ²³⁹ Np daughter grows into radioactive secular equilibrium with the ²⁴³ Am parent following a 2.4 day half-life for the ingrowth.

Then, the parent source is separated from the daughter. That is, Np and Am are easily separated chemically, allowing the ²⁴³ Am to be "milked" as known in the art as a source of the isotopically pure ²³⁹ Np. "Milking" is used to describe a separation of two isotopes that is followed by a further daughter isotope ingrowing back into the source thereof which is the daughter isotope as separated by the milking from the oriqinal parent. As described above, a daughter ²³⁹ Np is formed by the decay of the parent ²⁴³ Am, wherein the daughter Np is formed in the source by the decay of the parent Am.

In this case, the milking is preferably performed via an ion exchange column. More particularly, ²⁴³ Am is absorbed on an ion exchange column in which a change in acidity can be used, wherein the Np is washed out of the column and the Am remains in the column, thus providing the milking.

The ²³⁹ Np decays to the further daughter ²³⁹ Pu, allowing the ²³⁹ Np isotope to be used as a source of isotopically pure ²³⁹ Pu. That is, the Np daughter is electroplated using, e.g., a dimethylsulfoxide solvent on a suitable substrate such as a high purity metal; nickel is preferred. Electroplating techniques are known in the art for this purpose.

More particularly, ²³⁹ Np in an amount of 1.36×10⁵ dpm produced as described above is electroplated on a substrate. The decay rate can be easily characterized by known radiometric means. A waiting period of many ²³⁹ Np half-lives is allowed to pass, i.e., several weeks. As a result, 2.8×10⁻¹³ gram for instance of isotopically pure ²³⁹ Pu can be formed by the complete decay of the ²³⁹ Np isotopic spike.

Another embodiment of the method of the present invention will now be described, directed to producing isotopically pure ²³⁷ Np. According to this embodiment, isotopically pure ²⁴ lPu is used as the radioactive parent source for producing an isotopically pure ²³⁷ U daughter. That is, ²⁴¹ Pu decays according to the following Equation (2): ##STR3## The ²³⁷ U daughter grows into radioactive secular equilibrium with the ²⁴¹ Pu parent following a 6.75 day half-life for the ingrowth. U and Pu are easily separated chemically, allowing the ²⁴¹ Pu to be milked as described above as a source of isotopically pure ²³⁷ U. Since ²³⁷ U decays to ²³⁷ Np, the ²³⁷ U isotope is a source of isotopically pure ²³⁷ Np.

More particularly, ²³⁷ U in an amount for instance of of 1.8×10⁶ dpm is electroplated on a substrate. Again, the decay rate can be easily characterized by known radiometric means. The sufficient waiting period of several weeks, namely of many ²³⁷ U half-lives, is allowed to pass. ²³⁷ Np in the amount of 10⁻¹¹ gram is accordingly formed by the complete decay of the ²³⁷ U isotopic spike.

The only known disadvantage of the latter embodiment described above results from the fact that the ²³⁷ U spike must be chemically pure prior to each electroplating session, since the ²³⁷ Np formed by previous decays will also undergo electroplating. Conventional electroplating operating conditions would require an initial Np-U chemical separation on the ²³⁷ U spike material followed by production of the deposits in the ensuing few hours. This is a small disadvantage when compared with the method described supra in the Backgound of the Invention, namely, requiring ²³⁹ Np-²³⁷ Np spike equilibration.

The ultralow-mass fissionable deposits produced by this method can then be used as fissioning sources for solid state track recorder fission rate measurements in high intensity neutron fields to derive information for neutron dosimetry purposes.

More particularly, a solid state track recorder may be placed adjacent to a thin fissionable deposit produced by the present invention. The solid state track recorder then records tracks from the recoiling fission fragments which result from the fissions in the deposit. The fissionable deposit is then exposed to a neutron fluence for a predetermined period. The value of the neturon fluence can then be determined based on the number of fissions of the fissionable deposit. More particularly, the number of tracks formed by the fissionable deposit and observed with an optical microscope after chemical etching of the solid state track recorder is proportional to the number of fissions that has occurred in the fissionable deposit. Thus, the number of fission fragment tracks per square centimeter, namely, the track density in the solid state track recorder, is used to calculate the fission rate per unit area in the fissionable deposit.

The most important advantages of this method for producing ²³⁹ Pu and ²³⁷ Np deposits as compared to other methods are as follows:

(1) the decay rates are much higher, thus allowing easier deposit characterization;

(2) the resulting material in the deposit is isotopically pure;

(3) detector background is no longer a problem;

(4) chemical equilibrations of tracers or spikes are no longer necessary;

(5) extremely low masses, i.e., as low as 10⁻¹³ g for ²³⁷ Np and 10⁻¹⁶ g for ²³⁹ Pu, can be produced; and

(6) the time savings that accrue from using this method are substantial.

The foregoing is considered illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described. Accordingly, all suitable modifications and equivalents may be resorted to, as falling within the scope of the invention and the appended claims and their equivalents. 

I claim:
 1. A method for producing an isotopically pure, ultralow-mass fissionable deposit which can be accurately calibrated and used as a nuclear reactor dosimeter, comprising the steps of:(a) holding a radioactive parent until the radioactive parent reaches secular equilibrium with a daughter formed by the radioactive decay of the parent; (b) chemically separating the daughter from the parent; (c) electroplating the daughter on a substrate; (d) accurately determining the amount of daughter present by measuring its decay rate; and (e) holding the electroplated daughter until the daughter decays to the ultralow-mass fissionable deposit whose mass shall then be known since the amount of the daughter was previously determined.
 2. A fissionable deposit prepared by the method of claim 1, wherein the fissionable deposit is effectively isotopically pure.
 3. A method of using an electroplated fissionable deposit as a nuclear reactor dosimeter, comprising the steps of:(a) holding a radioactive parent until the radioactive parent reaches secular equilibrium with a daughter formed by the radioactive decay of the parent; (b) chemically separating the daughter from the parent; (c) electroplating the daughter on a substrate; (d) accurately determining the amount of daughter present by measuring its decay rate; (e) holding the electroplated daughter until the daughter decays to the ultralow-mass fissionable deposit whose mass shall then be known since the amount of the daughter was previously determined; (f) combining the fissionable deposit with a solid state track recorder; (g) exposing the fissionable deposit to a neutron fluence; and (h) determining the value of the neutron fluence based on the number of fissions of the fissionable deposit.
 4. A fissionable deposit prepared by the method of claim 3, wherein the fissionable deposit is effectively isotopically pure.
 5. The method as recited in claim 3 wherein step (a) comprises the substep of:choosing the parent from the group consisting of ²⁴³ Am and ²⁴¹ Pu.
 6. The method as recited in claim 3, wherein the daughter is ²³⁹ Np.
 7. The method as recited in claim 3, wherein the daughter is ²³⁷ U.
 8. The method as recited in claim 3, wherein the fissionable deposit is ²³⁹ Pu.
 9. The method as recited in claim 3, wherein the fissionable deposit is ²³⁷ Np. 